1. (Field of the Invention)
The present invention relates to a ceramic resistor comprising chiefly an aluminum nitride which is suited for being used as a heater material, enclosure of a vacuum tube, antistatic material in an apparatus for producing semiconductors, wafer-conveying arm, wafer-handling jig and, particularly, electrostatic chuck, as well as to an electrostatic chuck using the above resistor as a resistor layer.
2. (Description of the Prior Art)
The electrically insulating ceramics have heretofore been adjusted for their electric resistance by adding electrically conducting materials to the electrically insulating ceramics. For instance, titanium nitride is added to the alumina to decrease its electric resistance.
The aluminum nitride is a non-oxidizing ceramic which has been expected for its use as a structural material and a high-temperature material. In recent years, it has been reported that the aluminum nitride exhibits excellent resistance even against plasma. Therefore, study has been forwarded to use the aluminum nitride as a part such as an electrostatic chuck in an apparatus for producing semiconductors. However, the aluminum nitride is a highly insulating material exhibiting a resistivity of not smaller than 10.sup.16 .OMEGA.-cm even at room temperature, and has not been put into practical use.
In the apparatuses for producing semiconductors, an electrostatic chuck has been most widely used for holding and handling silicon wafers because of such advantages that it offers flat machining surfaces and degree of parallelism at the time of machining silicon wafers and that it makes it possible to machine the silicon wafers in vacuum.
Electrostatic chucks have heretofore been proposed such as the one having an insulating layer composed of alumina, sapphire, etc. formed on an electrode plate (Japanese Laid-Open Patent Publication No. 281377/1985), the one having an electrically conducting layer formed on an insulating substrate and having an insulating layer formed thereon (Japanese Laid-Open Patent Publication No. 34953/1992), the one having an electrically conducting layer incorporated in an insulating substrate (Japanese Laid-Open Patent Publication No. 94953/1987) and the like.
The trend toward an increase in the degree of integrating semiconductor elements on an integrated circuit in recent years has required an increase in the precision of the electrostatic chuck and, particularly, an increase in the corrosion resistance, wear resistance, heat resistance and shock resistance of the electrostatic chuck.
In particular, the ceramics used for such applications must have excellent resistance against plasma, and attempts have been made to use the aluminum nitride as an insulating layer for the electrostatic chuck since it has superior resistance against plasma to other ceramic materials.
As described above, however, the aluminum nitride itself is a highly insulating material having a resistivity of not smaller than 10.sup.16 .OMEGA.-cm. On the other hand, the insulating layer for the electrostatic chuck must have a resistivity of not larger than 10.sup.14 .OMEGA.-cm and, preferably, within a range of from 10.sup.7 to 10.sup.12 .OMEGA.-cm, especially 10.sup.8 to 10.sup.12 .OMEGA.-cm to enhance the efficiency for generating electrostatic force. Therefore, it is not allowed to use the aluminum nitride in its form as the insulating layer.
It has been attempted to decrease the electric resistance by modifying the aluminum nitride. For instance, Japanese Laid-Open Patent Publication No. 4509/1981 discloses a method of adjusting the resistivity by adding an electrically conducting material such as aluminum or the like to insulating ceramics such as aluminum nitride and boron nitride. As for thin-film ceramics, furthermore, Japanese Patent Publication No. 50884/1980 proposes a thin-film resistor having a small resistance-temperature coefficient by dispersing metal aluminum in, for example, the aluminum nitride.
However, the method of controlling the electric resistance by adding an electrically conducting material arouses a problem in that properties of the electrically conducting material impair the properties, such as resistance to plasma exposure, which is inherent in the insulating ceramics.
For instance, resistance to plasma and durability of the aluminum nitride may be lost, or properties of the aluminum nitride may be deteriorated.
In general, furthermore, the volume resistivity of an insulator tends to decrease with an increase in the temperature. In the case of the aluminum nitride, for instance, the resistivity which is 10.sup.16 .OMEGA.-cm at room temperature decreases down to 10.sup.11 .OMEGA.-cm at 300.degree. C. and further decreases down to 10.sup.7 .OMEGA.-cm at 600.degree. C.
This results in the occurrence of such an inconvenience that when the insulator or the insulator layer is used as an electrostatic chuck material, the resistance undergoes a change due to a change in the temperature making it difficult to obtain a stable absorbing force and, further, imposing limitation on the temperature range that can be used.
To avoid such inconvenience, Japanese Laid-Open Patent Publication No. 160444/1990 proposes a structure comprising two or more insulating layers that are stacked, electrode layers corresponding to the insulating layers, an electric circuit and a switch, which can be used over a temperature range of from room temperature up to 400.degree. C. Japanese Laid-Open Patent Publication No. 300187/1992 teaches an art in which a temperature detector such as heater or thermocouple is mounted in an electrostatic chuck, and a control unit is provided on an external side to control the power source unit depending upon a change in the temperature, in an attempt to stabilize the absorbing force and to broaden a range of temperature in which the electrostatic chuck can be used. However, when the number of electrode layers is increased by stacking two or more insulating layers as described above, the electric circuit becomes complex and, besides, the structure of the electrostatic chuck itself becomes complex. Accordingly, the step of production becomes complex, resulting in a decrease in the reliability of the products and an increase in the cost of production.
Even in a method which incorporates the heater, detects the temperature of the heater and controls the applied voltage, the electrostatic chuck can be no longer used if a defect occurs in the temperature detector such as thermocouple incorporated in the electrostatic chuck. Even according to this method, properties possessed by the ceramic material do not substantially change, and limitation is imposed on the range of use.
In order to cope with the above-mentioned problems, the present inventors have forwarded the study to realize a ceramic resistor having an electric resistivity of not larger than 10.sup.14 .OMEGA.-cm from the standpoint of its composition and structure, and have discovered the fact that there can be obtained an insulator having a volume resistivity of not larger than 10.sup.14 .OMEGA.-cm, exhibiting a small change in the resistivity relative to the temperature and stable properties over a wide range of temperatures by doping the insulator comprising chiefly the aluminum nitride formed by, for example, a chemical vapor-phase deposition method with a particular element in an amount over a predetermined range, and permitting the element to be solid-dissolved in the crystal of aluminum nitride so that the lattice constants of the aluminum nitride lie within predetermined ranges, and have thus arrived at the present invention.